1. Field
This invention concerns a content marking system, in particular in the domain of digital video recorders.
2. Description of Related Art
The introduction of personal digital recorders (PVR or DVR) for video content has opened the way to new applications such as the broadcasting of a pre-recorded content when desired.
This storage and reproduction ability offers the opportunity of the abusive use of the content. In fact, this content is generally under copyright protection and its reproduction subject to very strict conditions.
The content, as defined within the scope of this application, includes an audio part and a video part. It is sent to the recorders in an encrypted form by means of control words (CW) which are frequently changed.
This technique, initially developed for broadcasting, poses some problems when recording and reproducing. A known problem is the time displacement inside the content, i.e. fast rewind and fast-forward.
In this example, the encryption of the content does not allow the setting of the time and in particular finding the beginning of a group of images. A group of images includes a complete frame i.e. containing all the data included in an image and differential frames, i.e. containing information about differences in relation to the previous frame.
Fast forward and fast rewind need to be located in complete frames, also referred to as I-frames, to start a new reading. Differential frames are referred to as P/B type frames.
A first solution has been described in application WO 02/069638, which consists of encrypting type I and P/B frames in a different way. This solution is satisfactory for selectively increasing the security of the content without excessively delaying the deciphering period of the data.
If a digital video recorder is in use, the ideal is to have unscrambled I-frames in order to allow an easy displacement in the content. Nevertheless, although I-frames allow to access to the contents in a very partial way, leaving unscrambled images stored in the storage unit without protection is not desired.